Conserving a language is the key to fully grasp the identity of a culture.

Let’s look functionallyat what we are trying to communicate in this sentence:

[…something A…] is the key to [..something B..].

OK. So something A is obviously a thing or a behaviour that will help us to achieve something B, which is a thing or a behaviour that is difficult to achieve without something A. If something Bis a thing, then it is a noun and not a verb, so we need the noun form of grasp, which is grasping:

Conserving a language is the key to fully grasping the identity of a culture.

For some reason the expressions the key to and the answer to often lead to this error, so watch out for those! In this structure, ‘to’ is a preposition and not part of the infinitive ‘to + V1’.